Roads and other pavements formed of asphaltic concrete or the like require repair, which may include resurfacing, after a period of use. Overlaying such pavements with new asphaltic concrete is costly. Petroleum based asphalt is itself expensive and transporting new paving material from a distant hot mix plant or the like adds substantially to costs.
Economies may be realized by recycling the original paving materials on site. A roadway or the like may be resurfaced by heating the deteriorated pavement or at least the upper portion of the pavement to soften the asphalt binder and by then remixing and recompacting the heated material. Small areas containing cracks, potholes or the like may be repaired by an essentially similar technique.
It is highly advantageous to utilize microwave heating in such resurfacing or repair operations. Microwave energy instantly penetrates most paving materials, typically to depths of about 20 centimeters, and generates heat within the penetrated material in the process. The pavement is heated more deeply, rapidly and uniformly than is practical if older techniques, which apply externally produced heat to the pavement surface, are employed. Methods and apparatus for heating pavements in place with microwave energy are described in my prior U.S. Pat. Nos. 4,319,856; 4,175,885; 4,252,459 and 4,252,487.
In some paving resurfacing or repair operations it is not necessary to heat and remix the asphaltic pavement down to the full depth to which microwave energy penetrates into pavements. A more economical reworking of just the top several centimeters may be sufficient. Unnecessarily deep microwave heating of the pavement and/or underlying material in such circumstances unproductively consumes costly energy.
Known techniques for controlling the depth of heating in paving or paving repair operations are not effective in the case of microwave heating. Older heating techniques rely on the downward thermal conduction of externally generated heat that is applied to the pavement surface. Depth of heating is affected by adjustment of the rate at which such heat is applied to the pavement surface or by varying the length of time during which the heat is applied. Neither of these procedures is effective for controlling the depth of direct heating of paving material by microwave energy.
Microwave energy, which is not itself heat, is a form of electromagnetic energy that instantly penetrates into dielectric materials and is then converted to heat within the penetrated region of the material. The efficiency of this conversion, for microwave energy of a specific frequency, is dependent on the molecular structure of the penetrated material and is not significantly affected by other factors. Thus the depth of penetration of the microwave energy remains essentially the same regardless of the rate at which it is applied or the duration of the period during which it is applied. Varying the intensity of the microwave energy or varying the period during which it is applied changes the degree of heating but does not significantly change the penetration depth of the microwave energy.
Paving operations of the kind described above would be more efficient and economical if microwave heating could be concentrated or localized at a predetermined region thereby avoiding unnecessarily deep heating of the pavement or underlying material.
The present invention is directed to overcoming one or more of the problems set forth above.